Drilling using coiled tubing as a drill string was first implemented several years ago and hundreds of wells are now drilled every year with this technology. A review of the use of re-entry drilling using coiled tubing can be found in HILL, D, et al. Reentry Drilling Gives New Life to Aging Fields. Oilfield Review. Autumn 1996, p. 4-14. Coiled tubing drilling (CTO) shows many advantages compared to conventional drilling with jointed pipes, including:                The ability to operate in pressurized wells;        Fast tripping speeds;        The ability to circulate continuously while tripping and drilling;        The ability to be used in slim hole and through-tubing; and        Rig-less operation.        
However, despite significant development over the years, CTD has remained a niche application, with primary markets limited to thru-tubing re-entries wells, under balanced and slim hole drilling. This limited expansion is due to certain inherent disadvantages of CTD:                A relatively large tubing size is needed for drilling applications and only a small portion of the current global CT rig fleet is capable of handling such sizes;        The size and the weight of a typical spool of coiled tubing is sometimes too great for the hosting capacity of platforms on which it is used;        CTD requires surface-pumping equipment that is comparable in size to that used in conventional drilling; and        CTD can only have a limited reach in horizontal wells.        
These problems arise, in part, from the fact that the basic drilling process is the same as that used in a conventional, rig-based drilling system. This means that the drilling process produces cuttings of a size and volume that still require powerful (and therefore large) surface pumping units, and large diameter coiled tubing to handle the cuttings in the borehole.
Recent proposals for the use of downhole drilling systems for use with wireline drilling operations have resulted in the development of downhole control of the drilling process. This has been required to accommodate the use of downhole electric motors for drilling and the fact that the conveyance system (wireline cable) cannot provide any weight on bit or torque reaction. Such systems typically use downhole tractors to move drilling tools through the well and provide weight on bit for the drilling process. A number of tractors are known for use in a borehole environment, such as those described in U.S. Pat. Nos. 5,794,703; 5,954,131; 6,003,606; 6,179,055; 6,230,813; 6,142,235; 6,629,570; GB 2 388 132; WO 2004 072437; U.S. Pat. Nos. 6,629,568; and 6,651,747.
This invention aims to address some or all of the problems encountered with the prior art systems.